This invention generally relates to imaging systems, such as ultrasound systems. In particular, the invention relates to a system and method for generating images using multiple frequencies of echo signals.
Ultrasound imaging systems generate and transmit ultrasound signals. Typically, the transmitted ultrasound signal is a pulse with a center frequency within the 2 to 15 MHz range. The ultrasound signal propagates through a body, such as the chest cavity of a patient. The transmitted ultrasound signal reflects off structures within the body, such as blood cells or tissue boundaries. Some of the reflected signals, echo signals, propagate towards the transducer.
As the transmit signal propagates through and scatters within the body, additional frequency components are generated, such as at harmonics of the transmit frequency. These additional frequency components continue to propagate through and reflect off structures in the body. Echo signals having the same frequencies as the transmit signal and echo signals associated with the additional frequency components impinge on the transducer. The additional frequency components are caused by non-linear propagation and non-linear scattering or reflecting.
The echo signals are detected and processed by the ultrasound system. Conventional ultrasound systems filter or otherwise remove signals having harmonic or different frequencies than the transmit frequencies. Thus, echo signals resulting from linear propagation and reflection are detected by the ultrasound system.
The detected linear echo signals are used to create an image. For example, Doppler and B-mode image information are derived from the linear echo signals. The information is then used to generate two-dimensional or graphical displays.
As an alternative and as a discussed by Ted Christopher in xe2x80x9cFinite Amplitude Distortion-Based Inhomogeneous Pulse Echo Ultrasonic Imagingxe2x80x9d, IEEE Trans. UFFC 44(1), pp. 125-139 (January 1997), harmonic echo signals, signals resulting from non-linear propagation and scattering, are detected. The echo signals having the same or similar frequency band as the transmit signals are filtered or otherwise removed. An image, such as a B-mode image, is generated from the detected harmonic echo signals. Typically, the signal-to-noise ratio of the echo signals at harmonic frequencies may be acceptable for B-mode imaging, yet unacceptable for Doppler imaging. Various techniques for non-linear imaging are disclosed in U.S. Pat. Nos. 5,410,516 and 5,255,683.
The invention provides a method and apparatus for obtaining and displaying various ultrasound data. Information based on both harmonic and fundamental frequencies are acquired and displayed at substantially the same time. In one aspect, a method and system for acquiring data in an ultrasound system are provided. A first transmit signal is transmitted, and a linear echo signal is acquired in response to the first transmit signal. An interleaved second transmit signal is transmitted, either at the first frequency band or a second frequency band, and a non-linear echo signal is acquired in response to the second transmit signal.
In another embodiment, a method of acquiring data in an ultrasound system is provided. Data is obtained in response to a fundamental frequency. Interleaved with the data, other data is obtained in response to a harmonic frequency.
In yet another aspect of the invention, a method of displaying ultrasound information is provided. A first image responsive to non-linear echo signals is displayed. A second image responsive to linear echo signals is displayed with the first image.